1. Field of the Invention
The present invention relates to a polysulfone based hollow fiber membrane and a process for preparing the same.
A large number of researches on the material of a separation membrane which has selective permeability had been made, and celluloses, polyamides, polyacryls and polyvinyls have been developed for the membrane material and practically are in use. These materials, however, have drawbacks such as bio-compatibility, chemical resistance, and heat resistance. In order to prevent these drawbacks, studies on adopting resins used for engineering plastics as separation membrane have been made, and engineering plastic resin like polysulfone based resin is in use as a material of a separation membrane. Since the polysulfone based resin has excellent properties such as bio-compatibility, chemical resistance, heat resistance, flame resistance, and mechanical characteristics, it is widely used as a membrane material for hemodialysis, micro filtration, ultra filtration, reverse osmosis, and gas separation.
Generally speaking, polysulfones hollow fiber membrane is produced by spinning the spinning dope composed of polysulfones resin, organic solvent and additives such as water soluble polymer, and internal coagulating liquid into air through a biannular spinning nozzle to obtain an extrudate in the form of a hollow fiber, and then coagulate the said extrudate by external coagulating liquid.
In the process of producing the said spinning dope, m-cresol, N-methyl-2-pyrrolidone, dimethyl sulfoxide, dimethyl acetamide, dimethyl formamide and/or the mixture thereof are used for solvent.
Water soluble polymer is selected from polyethylene glycol, polyvinyl alcohol, polyvinyl pyrrolidone and/or the mixture.
Though water is usually used for both internal and external coagulating liquid, water solution comprising the said additives could be used for both internal and external coagulating liquid.
2. Description of the Prior Art
In respect to the membrane, in general, the most important two properties are (1) the capability of eliminating more than a certain molecular weight of a separation material (hereinafter called "separation capability"), and (2) permeability, but these two properties are not mutually compatible: if the separation capability goes up, then the permeability goes down relatively, and if the permeability needs to be increased, then the separation capability cannot but be lowered. Therefore, an important task of developing a membrane is to produce a membrane which has a certain separation capability and higher permeability at the same time.
First of all, in order to improve the permeability of a membrane, many large pores should be formed in the membrane. In addition, in order to elevate the separation capability of a membrane while it maintains a certain permeability, the size of the pores formed in the membrane must be similar.
Up to the present date, several methods, adding water soluble polymer, solvent and/or alcohol compound in the internal coagulating liquid and/or external coagulating liquid, have been in use for form pores in a membrane. These methods, however, cannot support the formation of similar size pores so as to meet the required certain level of separation capacity and permeability in a hemodialysis membrane.
In the mean time, if the said water soluble polymer added in spinning dope, remains quite a lot in the hollow fiber membrane after the spinning process, the permeability, without deterioration in separation capability, shall be improved because of the elevated hydrophilicity of the hollow fiber membrane. In fact, after the spinning process, most of the water soluble polymer leaks to the outside of the hollow fiber membrane during the coagulating and washing processes, and there remains a little in the inside of the hollow fiber membrane because of its high solubility to water.
U.S. Pat. No. 5,340,480 discloses that a water solution containing polyvinyl pyrrolidone shall be applied as an internal coagulation liquid in order to leave polyvinyl pyrrolidone (hereinafter called "PVP"), the water soluble polymer, in the surface of the hollow fiber membrane for improvement of permeability, but the efficiency is insignificant.
Japanese Patent Laid-Open No. 58-104940 discloses use of a cross linkage agent for fixing a water soluble polymer to the surface of a polysulfone hollow fiber membrane, but the process and operation is very complicated.
Japanese Patent Laid-Open Nos. 63-97205 and 63-97634 disclose use of heat treatment or radiant rays treatment for fixing a water soluble polymer to the surface of a polysulfone hollow fiber membrane, but the process is complicated and it requires expensive equipment.
Therefore, in order to improve the separation capability and permeability of the membrane at the same time, the following techniques have been required to be developed:
1. A technique to form similar size pores in the membrane, and
2. A technique simply and effectively to leave the water soluble polymer which was added in the spinning dope, in the hollow fiber membrane.
The present invention relates to a process for producing a polysulfone based hollow fiber membrane having an excellent separation capability, permeability and high water permeable coefficient by forming many similar-sized pores in a polysulfone based hollow fiber membrane and by leaving a lot of PVP, the water soluble polymer, in the inside of the membrane.